$M$-Ary Spread Multiple Access and Its Applications in Uplink Cellular Communications

More effective multiple access techniques are required to fulfill the pressing demand of supporting a large number of connected devices in future mobile communications. This paper proposes an <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$M$</tex-math></inline-formula>-ary spread multiple access (MSMA) scheme, designed to provide a high overload ratio for uplink cellular communications. The MSMA scheme distinguishes users by employing codes with superposition constellations. Specifically, it utilizes <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$M$</tex-math></inline-formula>-ary code mapping scheme to map each user's data stream onto several subcarriers, thereby achieving a frequency diversity gain. The receiver decodes incoming signals using <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$M$</tex-math></inline-formula>-ary code demapping based on generalized sphere decoding (GSD), which effectively balances diversity gain and multi-user interference, leading to enhanced performance. Both analytical and simulation results confirm that the MSMA system yields a lower bit error rate (BER) than power domain non-orthogonal multiple access (PD-NOMA) with the same overload ratio. In addition, MSMA surpasses sparse code multiple access (SCMA) in terms of both BER and overload ratio when the same number of subcarriers are reused. Therefore, with the same number of subcarriers and the same BER requirement, MSMA can support more users than SCMA and PD-NOMA.